1. Field of the Invention
This invention relates to a controller for a liquid crystal display which eliminates any errors or lowering of contrast, caused by the variation of a liquid crystal temperature. More particularly, it relates to a controller for a liquid crystal display which can prevent displaying errors to be caused by increase in a response time of the liquid crystal display at a low temperature.
2. Description of the Prior Art
A statically driven system using a segment type display will be explained hereinafter as one example, with respect to a waveform of a driving voltage applied to a conventional liquid crystal display and the changes of the image on its display screen.
Assume that the image on the display screen of the liquid crystal display is varying at a predetermined time interval, such as 6-7-5-1 - - - , as shown in FIG. 2.
A waveform of a driving voltage to be applied to S1 segment on the display screen is shown in FIG. 1(a). The driving voltage is applied across a transparent electrode at the front of the display panel and a common electrode at the back thereof. In this case, an alternating current driving voltage is applied thereto for a certain period, which has a square waveform and a predetermined frequency. During the application of its driving voltage, the S1 segment of the liquid crystal screen increases its light transmittance so that the S1 segment comes to be visible.
FIG. 1(b) illustrates a voltage response characteristic which shows the variation of light transmittance of liquid crystal in the S1 segment when the voltage of FIG. 1(a) is applied thereto, wherein the ordinate axis shows light transmittance, and the abscissa axis shows time. In this case, a reference level Th shows a threshold and if the light transmittance is in excess of Th, it can be recognized that the S1 segment is in a visible state.
As is apparent from the voltage response characteristic shown in FIG. 1(b), if the rise and fall times of the light transmittance are extremely short, as compared with a switching period of the image, the light transmittance can respond to the envelope of the driving voltage waveform with adequate accuracy.
In such a case, therefore, the visual condition on the display screen can be switched at a predetermined time interval as shown in FIG. 2, so that there is no problem of displaying errors.
On the other hand, however, liquid crystal has such a physical property that the responsiveness of its light transmittance to the driving voltage is lowered as its temperature comes down. In other words, either of the rise and fall times in the voltage response characteristic is increased in proportion to an exponential function of a reciprocal absolute temperature, and therefore, at an extremely low temperature, the voltage response characteristic can be shown by such a curve as illustrated in FIG. 1(c).
As is known from this characteristic figure shown in FIG. 1(c), the voltage response characteristic curve of light transmittance in liquid crystal cannot exceed the thereshold Th in the time block T2, although a driving voltage is applied thereto. This voltage response characteristic curve goes into the next time block T3 in which a driving voltage is not applied, before it exceeds the threshold Th.
Therefore, the S1 segment, which ought to be in a visible state in the time block T2, cannot be visible in this case. In the same manner as described above, it is evident that the S1 segment is not visible either, in the blocks T4 and T7.
Thus, since the voltage responsibility of the light transmittance is lowered at a low temperature, it is impossible for the visual condition to be changed at a predetermined period. As a result, display errors or lowering of contrast may be encountered.
As to liquid crystal, it has also been known that the rise time in the voltage response characteristic decreases in proportion to the value of a driving electric field applied in the inside of liquid crystal.
Taking notice of this fact, such a method can be supposed that under a low temperature, a voltage to be applied thereto is increased as compared with the voltage value in the case of a usual temperature thereof. However, the fall time in the voltage response characteristic is scarcely concerned with the effective value of the electric field inside of the liquid crystal. Therefore, even if a large amount of voltage is applied to operate the liquid crystal, the voltage response characteristic curve for its light transmittance is only as shown in FIG. 1(d).
In this case, in a time block T2 in which a voltage is applied, the light transmittance can sufficiently respond to the applied voltage within the predetermined time after a voltage application. In a time block T3, however, if the voltage is eliminated, the fall curve thereof cannot pass through the threshold Th within the time block T3, because the fall time thereof is more than twice as long as the predetermined period for switching a visual condition.
As a result, the S1 segment continues to be visible in the time block T3 though the display ought to disappear therein. Then, at the following time block T4, a voltage for display is also applied.
In this case, therefore, the visible state of the S1 segment is continued in all of the respective time blocks T2, T3, T4, T5, T6, T7 and T8 with no disappearance, and it is impossible to change the visual condition of S1 segment at the same period with that of the foresaid time block.
Thus, under a low temperature, even if the peek value of the applied voltage is increased, such a displaying error cannot be improved.
Accordingly, if the voltage is applied and stopped for the same period, such a display error, as described above in the prior art, is transitionally caused at the time when a display is switched, because of the difference between the rise time and the fall time of the voltage response characteristic curve. On the other hand, if the display is switched within the shorter period than the fall time in the voltage response characteristic, the aforesaid display error will necessarily be also caused.